This is a Fast-Track proposal to develop a tissue-based genetic test for BRCA1 hereditary ovarian cancer. Ten percent of unselected US ovarian cancers are BRCA1 hereditary cancers with truncating mutations in the BRCA1 gene. It is important to know which ovarian cancer patients have BRCA1 hereditary ovarian cancer because BRCA1 patients have an increased risk of breast cancer and their relatives with mutations have an increased risk of breast and ovarian cancer. These subsequent cancers would be discovered early or prevented if widespread genetic screening was available and recent data indicates that BRCA hereditary cancers respond to targeted therapies such as cis-platinum and PARP inhibitors. We have developed an antibody-based tissue truncation method to identify BRCA1 hereditary cancers and were able to correctly identify 5/5 BRCA1 hereditary cancers and 20/200 sporadic cancers in a pilot study. This approach visualizes protein truncation by showing with immunohistochemistry (IHC) that the N-terminus is present but the C-terminus is absent. The product (test) will be an antibody-based IHC diagnostic kit for cancer tissue samples which hospital labs will use to identify hereditary cancers. We have already successful developed and published a similar IHC method to identify BRCA2 hereditary cancers with sensitivity of 95% and specificity of 98%.The milestones for the Phase I proposal are: 1) Determine optimal tissue preparation protocols for IHC with both N-terminal and C-terminal BRCA1 antibodies. 2) Develop a quantitative scoring system for N-terminal and C-Terminal BRCA1 immunostaining on ovarian tissues with intra-assay variability of 15% and inter-assay variability of 20% or less. 3) Demonstrate that a C-terminal to N-terminal ratio for protein truncation (BRCA1 Truncation Ratio) can distinguish 20 hereditary BRCA1 ovarian cancers from 50 sporadic ovarian cases with greater than 90% sensitivity and specificity (comparing different scoring systems). Upon completion of these milestones we would pursue the following Phase II milestones: Milestone 1a: Determine the specificity of the BRCA1 truncation ratio in 20 ovarian cancers in patients known to have BRCA2 mutations. Milestone 1b: Determine the specificity of the truncation ratio in 10 ovarian cancers in patients reported to have polymorphisms/missense variants of BRCA1. Milestone 2: Determine if the BRCA1 truncation test can achieve 90% sensitivity and specificity in a clinical trial of 300 ovarian cancer samples (sample size to provide 30 BRCA1 hereditary cancers). The medical application of breast/ovarian cancer genetic testing is a proven market since Myriad Genetics annually grosses $150 million on DNA sequencing.